This invention relates to a high-power RF transmission line switch, and more particularly to a single-pole, double-throw (SPDT) switch.
Many applications require a high power RF (e.g., 1.2 GHz) switch having low volume and high power (0.5 to 5 kW) handling capability under vacuum or near vacuum conditions. Space missions are typical applications. For example, assume a spacecraft is to be equipped with a synthetic aperture radar transmitter for wave height or surface roughness measurements, as described in a copending application Ser. No. 744,577 filed by the Administrator of the National Aeronautics and Space Administration in respect to an invention of Atul Jain. Further assume the radar transmitter is to operate with an output power of 800 watts peak with redundance in the output power amplifiers in case of power amplifier failure. Output amplifier selection may then be accomplished with coaxial SPDT switch.
Commercially available coaxial SPDT switches provide a hermetically-sealed enclosure for the switch contacts in order to fill space around the contacts with an inert gas, such as N.sub.2. However, these switches have been found to fail during high power test operation in a vacuum, which indicates that these switches would very likely fail in space. The reason for the failure is that the hermetic seals are designed for operation in the earth's atmosphere where the pressure outside the enclosure is very nearly the same as the inert gas inside. With a very low pressure or vacuum outside, the switch enclosure tends to leak. Once all or most of the inert gas escapes, the high power RF being switched will cause the contacts to be subject to multipactor breakdown, a phenomenon to be described hereinafter. That breakdown will cause the contacts to erode. What would be required is a new enclosure design in which loss of the hermetic seal leading to insulation breakdown at critically low pressure will not occur. However, because of the inherent unreliability of hermetic seals of even the most elaborate designs, it would be preferable to use a vented enclosure for a switch that is otherwise designed to withstand power levels up to 5 kW under vacuum conditions without multipactor breakdown followed by ionization.
"Multipactor breakdown" is an electron resonance phenomenon which can only take place in a vacuum, and is dependent upon the RF frequency and conductor spacing. The mechanism of the phenomenon is the release of an electron into the vacuum space due to the potential gradient between the conductors first in one direction and then, as the RF signal changes polarity, in the other direction. When the spacing and frequency are complementary the electron resonance phenomenon develops very quickly. The electron density in the space has to be low in order for the electrons to accelerate to sufficient velocities (without collision) to cause secondary emission, but a vacuum assures that condition so multipactor breakdown occurs, and once it does occur, ionization breakdown follows.